This invention relates to ionic polymers, and to methods for the preparation thereof.
Certain ionic polymers, often called ionomers, are well-known as thermoplastic materials which, because of their toughness and adhesive character, are useful as coatings for a wide variety of substrates and as molded materials such as golf balls. Such ionomers are described in U.S. Pat. Nos. 3,264,272; 3,404,134; 3,657,201; and 3,836,511.
These ionic polymers, while generally thermoplastic and often soluble in organic solvents, exhibit characteristics similar to those of cross-linked polymers at normal temperatures. At elevated temperatures, they are readily extruded or molded in the same manner as thermoplastic polymers. These ionic polymers are believed to exhibit cross-linked characteristics as a result of ionic bonding between molecules of the polymer. Usually such bonding exists as a result of ionic attraction between anionic carboxylate, sulfonate or phosphonate moieties covalently bonded to the polymer molecules and metal cations characteristic of salt moieties.
Such ionic polymers are conventionally prepared by first copolymerizing small quantities of vinyl acid comonomers with major amounts of non-polar vinyl monomers such as ethylene or styrene. The resulting acid copolymers are then neutralized with bases in order to incorporate the desired cation into the polymer and to provide the desired ionic cross-linking. Alternatively, the ionic polymers are prepared by a post-reaction process wherein the preformed polymer dissolved in an organic solvent is first reacted with sulfonating agents such as sulfur trioxide, acetyl sulfate or other reagents to introduce sulfonic acid groups onto the polymer backbone, and then neutralized with base to form the desired ionic cross-linkages. Unfortunately, both processes are multi-step, expensive, involve the use of highly corrosive acidic reagents and/or are very time consuming. Moreover, in the post-neutralization step which involves mixing the sulfonated polymer with more polar neutralizing agents in a mechanical device, the viscosity of the mixing increases considerably and thus results in a certain amount of polymer molecular weight degradation and non-uniform neutralization.
Attempts to overcome these deficiencies by direct copolymerization of non-polar monomers with ionic comonomers have not been totally successful. Usually such processes involve the use of expensive polar solvents or cosolvents for the non-polar monomer and ionic comonomer which must be recovered from the resulting polymer. Such solvents often cause chain-transfer reactions during polymerization resulting in lower molecular weight polymers.
Preparation of ionic copolymers via aqueous emulsion polymerization methods involves the use of water-soluble surfactants which have to be removed from the resulting ionic copolymers. Moreover, as a result of different solubilities of the non-polar monomer and ionic monomer in the aqueous phase of the emulsion, the incorporation of the ionic monomer into the ionic copolymer is non-uniform thereby producing a heterogeneous product instead of the desired homogeneous ionic polymer.
Thus, a single step, inexpensive process for making ionic polymers exhibiting the desirable properties of conventional ionomers including the physical properties of the most desirable high molecular weight, homogenous ionomers is needed.